Essential Chemistry: Laws, Atomic Models, and Chemical Bonds

Ponderal and Volumetric Laws

Ponderal Laws

• Law of Conservation of Mass: In a chemical reaction, the chemical interplay throughout the system complies with the rule that the mass of the reactants matches the mass of the obtained products.
• Law of Definite Proportions: When two elements combine to form only one type of compound, they always do so in a fixed, definite proportion by mass.
• Law of Multiple Proportions: When two elements combine to give rise to various compounds, while the amount of one element remains constant, the amount of the other varies in a ratio of simple whole numbers.

Volumetric Laws

• Law of Combining Volumes: The volumes of reacting gases and the obtained gaseous products maintain a simple ratio of whole numbers.
• Avogadro's Hypothesis: Equal volumes of different gases, under the same conditions of temperature and pressure, contain the same number of particles.

Atomic Structure and Models

Electrons are fundamental particles that are found in all atoms. Over time, several models have described their behavior and arrangement:

Thomson Model

This model describes the atom as electrically neutral. It imagines atoms as solid spheres of uniform positive charge neutralized by electrons, which are embedded within them (like a plum pudding).

Rutherford Model

Based on an experiment where a beam of alpha rays was passed through a very thin layer of gold foil. Upon impacting the foil, most rays crossed it and impacted a screen where flashes occurred, while others were diverted. This revealed that:

• The nucleus occupies a very small fraction of the total volume of the atom, yet it accounts for most of the atomic mass and contains the positively charged protons.
• The cortex (or shell) is the outer area where electrons orbit the nucleus, meaning the atom is mostly empty space.

Bohr Atomic Model

This model states that:

• Electrons orbit the nucleus only in certain stable circular orbits where their movement does not lose energy.
• The only allowed orbits are those that adopt a particular energy level.
• The further the orbit is from the nucleus, the greater its energy level.
• An electron can transition to a lower level by emitting the excess energy as electromagnetic radiation.

Chemical Bonds and Structures

Ionic Bond

Compounds with ionic bonds exhibit the following properties:

• At room temperature, they are crystalline solids.
• They have high melting points (MP) and boiling points (BP).
• They are hard and brittle.
• Many are soluble in water.
• They do not conduct electricity as solids, but they do conduct electricity when molten (liquids) or in aqueous solutions.

Covalent Bond (Molecules)

Covalent molecular substances exhibit the following properties:

• At room temperature, most are gases, a few are liquids, and very few are soft solids.
• They have low melting points (MP) and boiling points (BP).
• Polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents.
• They do not conduct electricity.

Covalent Crystals

Covalent network crystals exhibit the following properties:

• At room temperature, they are all crystalline solids.
• They have very high melting points (MP) and boiling points (BP).
• They are harder than ionic crystals.
• They are insoluble in any solvent.
• They do not conduct electricity.

Metallic Bond

Metallic substances exhibit the following properties:

• At room temperature, they are all solids (except mercury).
• They have high density, high melting points (MP), and high boiling points (BP).
• They possess a characteristic metallic luster.
• They are ductile and malleable.
• They are insoluble.
• They conduct electricity and heat efficiently.